The present application relates to the magnetic resonance imaging arts. It finds particular application in conjunction with a well-known rapid imaging technique known as echo planar imaging (EPI) and echo volume imaging (EVI) and will be described with particular reference thereto.
As described by Peter Mansfield in "Multi-Planar Image Formation Using NMR Spin Echoes", J. Phys. C., Vol. 10, pp 150-158 (1977) echo planar imaging rapidly induces a series of echoes following a single RF pulse. More specifically, an RF pulse and a slice select gradient are applied to excite resonance in a selected slice and a phase encode gradient is applied to phase encode the resonance. A series of read gradients of alternating polarity are applied immediately following each other. During each read gradient, a magnetic resonance signal or echo is read out. Between each read gradient, a short pulse or blip along the phase encode gradient axis is applied to increment the phase encoding of the resonance in the selected slice. A one-dimensional inverse Fourier transform of each echo provides a projection of the spin distribution along the read axis. A second inverse Fourier transform along the phase encoded echoes provides a second dimension of spatial encoding. Typically, the phase encode gradient blips are selected of an appropriate magnitude that data for a complete field of view is taken following each RF pulse. The total sampling time is determined by the number of sampled points per read gradient and the number of phase encode gradient steps.
As set forth in published U.K. Patent Application No. 2,208,718, Peter Mansfield has extended the above-described rectilinear echo planar imaging techniques to multiple planes. After performing the above-described echo planar imaging sequence, a pulse or blip along the slice select axis is applied. The slice select blip steps the phase encoding along a slice select axis, e.g. a z-axis. Thereafter, phase encode gradient blips are applied between each read gradient to step along the second plane in the phase encode direction. Because the phase encode blips in the first plane move the phase encoding to one extreme edge of the field of view, the phase encoding blips in the second slice are of the opposite polarity to step the phase encoding back in the opposite direction. In this manner, the multiple planes are aligned, but offset in steps in the z-direction.
Although the rectilinear echo planar imaging and the rectilinear echo volume imaging techniques of Mansfield have been proven successful, they have a significant drawback. In particular, each of the phase encode gradient blips and each of the slice select gradient blips induces eddy currents. The eddy currents cause ghosting and other artifacts commonly attributed to eddy currents in the resultant images. Another disadvantage of the rectilinear echo volume imaging is that the trajectory through k-space is reversed in time for alternate phase encode lines or "views". This causes phase discontinuities which can result in ghosting. Another problem with both the rectilinear echo planar and volume imaging resides in the gradient duty cycle. In general, most of the gradient duty cycle is concentrated on the read axis, with only small blips on the other axes. This dumps most of the gradient duty cycle load on a single set of amplifiers.
In spiral echo planar imaging techniques described in U.S. Pat. No. 4,307,303 issued December 1991 to Likes, and in the article "High-Speed Spiral-Scan Echo Planar NMR Imaging I", IEEE Trans. on Med. Imaging, Vol. MI-5, No. 1, pp 1-7 (March 1986) of Ahn, et al., these problems are mitigated. In particular, in the spiral echo planar imaging technique, the applied x and y-gradient pulses, i.e. along the traditional read and phase encode axes, are sinusoidally varying and linearly increasing. In this manner, data sampling commences at the center of the field of view and spirals outward, covering the field of view along a spiral trajectory. The spiral trajectory has several advantages including that it has a circularly symmetric point spread function as opposed to the non-isotropic point spread function of rectilinear echo planar imaging. Any artifacts in the spiral echo planar imaging from gradient eddy currents or a field inhomogeneity cause a rotational misregistration of the object; whereas, in rectilinear echo planar imaging, the time reversal of alternate views introduces a discontinuous phase error that results in ghost images. The spiral echo planar imaging also has a T2 weighting that is uniform in all directions; whereas, in rectilinear echo planar imaging, the T2 weighting is different in the x and y-directions.
Further, in spiral echo planar imaging, the gradient duty cycle is reduced relative to the gradient duty cycle required in rectilinear echo planar imaging. In spiral echo planar imaging, the gradients are linearly increasing on both axes. In rectilinear echo planar imaging, the gradient at full amplitude switches polarity or oscillates while the gradient along the other axis is only small blips. Further, in spiral echo planar imaging, the k-space trajectory varies more slowly and smoothly than in rectilinear echo planar imaging. The blips can again induce unwanted errors due to non-ideal gradients. Eddy currents or delays in the blipped gradient pulse introduce unwanted errors in the k-space trajectory in the region of the blips.
One of the drawbacks of the spiral echo planar imaging is that it is a single slice technique. To obtain multiple slices, the spiral echo planar imaging technique is repeated multiple times. An RF excitation pulse and slice select gradient followed by sinusoidally varying and linearly increasing x and y-gradients are applied for each slice.
The present invention contemplates a new and improved volume imaging technique in which k-space is sampled along curved paths such as spirals or circles.